It is well known that acrylic fibrous materials when subjected to heat undergo a thermal stabilization reaction wherein the fibrous material is transformed to a black form which is non-burning when subjected to an ordinary match flame.
Such modification generally has been accomplished by heating the acrylic fibrous material in an oxygen-containing atmosphere. It is believed that the resulting thermal stabilization reaction involves (1) an oxidative cross-linking reaction of adjoining molecules, (2) a cyclization reaction of pendant nitrile groups to a condensed dihydropyridine structure, and (3) a dehydrogenation reaction. The cyclization reaction is exothermic in nature and must be controlled if the fibrous configuration of the acrylic polymer undergoing stabilization is to be preserved. The thermal stabilization reaction heretofore has generally been believed to be diffusion controlled and to require considerable time for oxygen to enter the interior portions of the fiber.
On a commercial scale the thermal stabilization reaction commonly is carried out on a continuous basis with a continuous length of a multifilament acrylic fibrous material being passed in the direction of its length through a thermal stabilization zone which is provided with a heated gaseous atmosphere. The movement of the continuous length of acrylic fibrous material through the stabilization zone containing the heated gaseous atmosphere may be directed by rollers situated therein. Additionally, it has been proposed to internally heat the rollers which contact the acrylic fibrous material.
Representative United States patents which concern the thermal stabilization of an acrylic fibrous material include: U.S. Pat. Nos. 3,285,696; 3,539,295; 3,699,210; 3,826,611; 3,961,888; 4,186,179; and Reissue No. 30,414. Since the thermal stabilization reaction has tended to be unduly time consuming various routes have been proposed to expedite the desired reaction through some form of catalysis and/or chemical modification of the acrylic fibrous precursors. See, for instance, the following U.S. Pat. Nos. which are representative of this approach: 3,592,595; 3,650,668; 3,656,882; 3,656,883; 3,708,326; 3,729,549; 3,813,219; 3,820,951; 3,850,876; 3,923,950, 4,002,426; and 4,004,053.
The resulting acrylic fibrous materials can be used in the formation of non-burning fabrics. Alternatively, the stabilized acrylic fibrous materials can be used as precursors in processes for the formation of carbon or graphitic carbon fibers. U.S. Pat. Nos. 3,775,520 and 3,954,950 disclose representative overall processes for forming carbon fibers beginning with an acrylic precursor.
There has remained a need for a simple expeditious process for the formation of thermally stabilized acrylic fibrous materials. Such need is particularly acute in the overall context of carbon fiber production since the carbonization or carbonization and graphitization portions of the overall process commonly require a considerably lesser residence time than the initial thermal stabilization portion of the process. Accordingly, heretofore it has been essential to provide extremely large ovens in order to accomodate the acrylic fibrous material undergoing thermal stabilization if the entire process is carried out on a continuous basis with the fibrous material passing directly from the stabilization zone to the carbonization zone.
It has heretofore been proposed that a previously stabilized acrylic fibrous material may be carbonized and/or graphitized in a laser beam while present in a non-oxidizing atmosphere. See, for instance, U.S. Pat. No. 3,699,210; British Pat. No. 1,241,937; and German Offenlegungsschrift No. 1,945,145.
It is an object of the present invention to provide an improved process for the thermal stabilization of acrylic fibrous materials.
It is an object of the present invention to provide an improved process for the thermal stabilization of an acrylic fibrous material which surprisingly can be carried out on an expeditious basis, and which requires a lesser residence time than prior art processes wherein heat is transferred to the acrylic fibrous material primarily by convection.
It is an object of the present invention to provide an improved process for the thermal stabilization of an acrylic fibrous material which can be carried out without the excessive usage of energy as commonly required in the prior art.
It is an object of the present invention to provide a simple improved process for the thermal stabilization of an acrylic fibrous material which does not require the heating of the gaseous atmosphere surrounding the acrylic fibrous material.
It is an object of the present invention to provide an improved process for the thermal stabilization of an acrylic fibrous material wherein oxygen readily enters the interior of the acrylic fibrous material without any substantial formation of a diffusion limiting skin on the outer surfaces of the fibers during the course of the thermal stabilization reaction.
It is another object of the present invention to provide an efficient process for the stabilization of an acrylic fibrous material immediately prior to the carbonization or carbonization and graphitization of the same.
It is a further object of the present invention to provide an improved thermal stabilization process wherein the fibers are intermittently heated without the necessity that the fibers contact heated surfaces such as heated rollers which possibly may cause damage to the same.
These and other objects of the invention, as well as its scope, nature, and utilization will be apparent to those skilled in the art from the following detailed description and appended claims.